This invention addresses the receiver design for digital communication systems, specifically the equalization portion of the receiver. As an example, this invention has been applied to the EDGE standard (“Digital Cellular Communication System (Phase 2+) (GSM 05.01–GSM 05.05 version 8.4.0 Release 1999)”). The EDGE standard is built on the existing GSM standard, using the same time-division multiple access (TDMA) frame structure. EDGE uses 8-PSK (Phase-shift keying) modulation, which is a high-order modulation that provides for high data rate services. In 8-PSK modulation, three information bits are conveyed per symbol by modulating the carrier by one of eight possible phases.
A wireless channel is often temporally dispersive. In other words, after a signal is transmitted, a system will receive multiple copies of that signal with different channel gains, at various points in time. This time dispersion in the channel causes inter-symbol interference (ISI) which degrades the performance of the system.
FIG. 1 shows a prior art example of a multipath channel profile where the channel is characterized as being minimum-phase. The main signal cursor 102 is followed in time by post-cursors 104, 106, 108, and 110, each having progressively lesser energy than the main cursor. FIG. 2 shows a multipath channel profile characterized as being maximum-phase, where the main signal cursor 202 being followed by post-cursor energy rays 204, 206, 208, and 210, which are greater in energy than the main signal 202.
To combat the effects of ISI at the receiver, many different types of equalization techniques can be used. One popular equalization technique uses a Decision Feedback Equalizer (DFE). The DFE cancels the extraneous multipath components to eliminate the deleterious effects of ISI. A DFE is relatively simple to implement and performs well under certain known circumstances.
A DFE typically performs well over a minimum-phase channel, where the channel response has little energy in its pre-cursors and its post-cursor energy decays with time. A DFE typically consists of a feed-forward filter (FFF) and a feedback filter (FBF). The FFF is typically used to help transform the channel into such a minimum-phase channel.
Accordingly, for channels with maximum-phase characteristics, it is often the case that the channel response after the FFF still contains a significant amount of the energy in its post-cursors, and the performance of the DFE can degrade over these channels. What is therefore needed in the art is an equalizer, of the DFE type (or like simplicity), which can enhance the performance of the DFE over both minimum and maximum phase channels.